Optical element with periodic structure

ABSTRACT

An optical element comprising a periodic structure in which refractive index is distributed periodicly and a deforming portion which mechanically deforms by external action, wherein the deforming portion is integrally arranged with the periodic structure along the periodic direction of the periodic structure, and is so constructed as to change the periodicity of the periodic structure by deforming in the periodic direction of the periodic structure. A periodicity of the periodic structure (photonic band structure) in which the refractive index changes periodically can be controlled with a simple configuration.

TECHNICAL FIELD

The present invention relates to an optical element having a periodicstructure, and more particularly to a method for controlling theperiodicity of a multi-dimensional periodic structure showing a periodicchange in refractive index and an optical element comprising means whichcontrols the periodicity of such periodic structure.

BACKGROUND ART

Recently, a new artificial crystal called “phototonic crystal”, in whichmaterials of different refractive indexes are arranged periodicly with apitch equivalent to wavelength, is proposed and is attracting attention(E. Yablonovitch, Phys. Rev. Lett., 58(1987) 2059-2062). Activeresearches and developments are being made on such artificial crystalfor an application as an optical element, since it has an opticalinhibition band (photonic band gap) resulting from the so-calledphotonic band structure similar to a band structure in a semiconductor,and it also has a specific effect resulting from an apparent abnormalityin the refractive index (Japanese Patent Application Laid-Open No.2000-066002).

Because of such background, a technology for precisely controlling theperiodicity of the artificial crystal is becoming important forcontrolling the photonic band structure.

In such technical field, there has been proposed a method of positioningactuators around a fiber diffraction grating and extending orcontracting such actuators to apply a tension to the fiber therebycontrolling the distribution of refractive index within the fiber (cf.Japanese Patent Application Laid-Open No. H10-253829).

Also there has been proposed a method of introducing a substance ofwhich the refractive index or the transmittance is externallycontrollable (for example a piezoelectric element) into the crystal, andcausing elongation or contraction in such substance or a change of thecharacteristics thereof, thereby disturbing the periodicity of thecrystal (cf. Japanese Patent Application Laid-Open No. 2001-091911).

Also there has been proposed a method of applying an external pressureto the photonic crystal thereby controlling the pitch of a lattice (cf.WO 02/27384).

However, these prior technologies are associated with the followingdrawbacks.

The method of extending or contracting the optical fiber changes aone-dimensional periodic structure arranged in the incident direction oflight, and requires a member for generating an extending-contractingforce, such as a piezoelectric element, and also a transmission memberfor transmitting such force to the fiber, and control accuracy of thelattice pitch is influenced by the material, arrangement, connectionstate etc. of such transmission member.

Also the aforementioned apparent abnormality in the refractive indexappears in a periodic structure of two or more dimensions, and theapparatus becomes more complex in order to apply forces in two or moredirections through the transmission member.

Also the method of incorporating means for disturbing the crystalstructure within the photonic crystal is associated with drawbacks thatthe manufacture is complex, requiring a large number of process stepsand that the usable material is considerably limited.

Also in the method of applying an external pressure to the photoniccrystal for varying the crystal structure thereof, it is necessary, asshown in FIG. 8, to support a photonic crystal 602 and a piezoelectricelement 603 with a support member 601 in surrounding manner.Consequently the apparatus becomes bulky.

Therefore, the present invention is to provide a method for controllinga periodic structure, capable of solving the aforementioned drawbacksand enabling to control a periodic structure which shows a periodicchange in the refractive index (photonic band structure) with a simpleconfiguration, and an optical element having periodic structure controlmeans.

DISCLOSURE OF THE INVENTION

The present invention is constructed as follows.

Firstly, an optical element of the present invention includes a periodicstructure in which refractive index is distributed periodicly, and adeforming portion which is mechanically deformed by action from theexterior, and is characterized in that such deforming portion isintegrally arranged with the periodic structure along a direction ofperiodicity of the periodic structure in such a manner as to change theperiodicity of the periodic structure by deformation in the direction ofperiodicity of the periodic structure.

Such change in the periodicity is a change in the period, phase, duty,orientation or combination thereof.

The optical element of the present invention has a property ofreflecting an incident light having a wavelength within a predeterminedrange and transmitting the other light. A light inside the opticalelement propagates in the region of periodicity of the aforementionedperiodic structure. The aforementioned deforming portion is preferablypositioned outside such light propagating region so as not to interceptthe light propagation.

The deforming portion is preferably a member integrally adjoined to theperiodic structure or is formed by the same member as the periodicstructure, and supports the periodic structure and deforms in thedirection parallel to the joint interface or boundary plane with theperiodic structure.

The optical element of the present invention causes mechanicaldeformation in the deforming portion by an electrical, mechanical orother external force, and is applicable, utilizing a resulting change inthe aforementioned optical property, to a mirror having a variablereflecting direction or a light deflector causing a change of the angleof a light exit direction with respect to a light incident direction.

Also a control method for an optical element of the present invention isa method for controlling an optical element including a periodicstructure in which the refractive index is distributed periodicly,characterized by arranging a deforming portion, which is mechanicallydeformed by action from the exterior, integrally with the periodicstructure along the direction of periodicity of the periodic structure,and causing deformation in the direction of periodicity of the periodicstructure, thereby changing the periodicity of the periodic structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view explaining an optical element in a first embodiment ofthe present invention.

FIG. 2 is a view explaining an optical element in a second embodiment ofthe present invention.

FIGS. 3A and 3B are views explaining an optical element in an example 1of the present invention.

FIGS. 4A and 4B are views explaining another configuration of theoptical element in the example 1 of the present invention.

FIGS. 5A and 5B are views explaining an optical element in an example 2of the present invention.

FIGS. 6A and 6B are views explaining another configuration of theoptical element in the example 2 of the present invention.

FIG. 7 is a view showing an example of configuration of a mirrorutilizing an optical element embodying the present invention.

FIG. 8 is a view showing a prior example.

FIGS. 9A, 9B, 9C, 9D and 9E are views showing examples of deformation ofthe optical element of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained with reference tothe accompanying drawings. In the following explanation of the drawings,including that of examples, the same components are indicated by thesame symbol.

FIG. 1 is a view showing an element configuration of an optical element,for explaining a first embodiment of the present invention.

As shown in FIG. 1, the optical element of the present embodiment isconstituted of a photonic crystal (hereinafter represented as PC) 101,and a substrate 102. The PC 101 has a multi-dimensional periodicstructure showing a periodic change of the refractive index. A crystalstructure having such multi-dimensional periodic structure is notparticularly restricted as long as there is formed a photonic bandstructure capable of suppressing light propagation.

The PC 101 can be prepared by an already reported and known method, suchas a lithographic technology, an etching technology, a self-formingmethod such as an opal method, or a micromachining technology, and themethod of preparation is not particularly limited. The substrate 102 isconstituted of a substance which changes its shape by externally appliedenergy. After the preparation of the PC 101, the PC 101 is closelyadhered to the substrate 102 thereby obtaining the optical element.

As explained in the foregoing, the optical element of the presentembodiment is constituted in a state where the PC 101 is integrated onthe substrate 102. Therefore, when the substrate 102 causes deformation(mechanical deformation) by externally applied energy, the PC 101correspondingly deforms in shape integrally with the substrate 102. Thusthe substrate constitutes a deforming portion for mechanically deformingthe periodic structure, and deformation in the substrate itselfintegrally changes the periodicity of the periodic structure.

Such deformation in the shape of the PC 101 integral with the mechanicaldeformation of the substrate allows to change the lattice shape orlattice pitch in the crystal structure (multi-dimensional periodicstructure). Such change in the lattice shape or in the lattice pitchincludes not only a change in the pitch, or the period itself, of thelattice but also a case of deforming the shape of individual lattices.The change in the lattice shape includes the change of the phase of theperiodic structure and the change in distribution of refractive indexwithin a period, namely the duty. It is also possible, the will beexplained in the following, to change the orientation of the lattice bygiving shear deformation thereto.

A light entering one end of the PC 101 is reflected in case thefrequency is within the inhibition band, but proceeds along the periodicstructure in case the frequency is outside the inhibition band. In theabsence of an anomaly in the refractive index, the light is emitted fromthe other end of the PC, while in the presence of an anomaly in therefractive index, the proceeding direction is changed but the light pathremains within a plane of the same periodic structure. In either cases,the deforming portion (substrate 102), being positioned outside theplane of the two-dimensional periodic structure, does not hinder thelight path. Therefore, it need not be formed of a transparent member,and also in case it is constituted of a piezoelectric member, anelectrode material need not be transparent.

Since the frequency of the light corresponding to the photonic bandstructure can be determined from the lattice shape and the lattice pitchmentioned above, such change in the shape of the PC 101 allows to changethe lattice shape or the lattice pitch thereby controlling the frequencycharacteristics. FIGS. 9A to 9E show examples of the deformation in theperiodic structure of the PC 101 in the present embodiment. FIGS. 9A to9E are plan views of the optical element shown in FIG. 1, showing atwo-dimensional periodic structure within a plane, in which therefractive index within circular cylinders arranged in a tetragonallattice is different from that in the surrounding area. It is assumedthat the light enters the PC 101 from the left-hand side of the drawing.

FIG. 9A shows a state without deformation; FIG. 9B shows a stateelongated in the x-direction; FIG. 9C shows a state contracted in they-direction; FIG. 9D shows a state with a shear deformation in thex-direction; and FIG. 9E shows a state with a shear deformation in they-direction. In FIGS. 9B and 9C, the lattice pitch or the period ischanged respectively in the x-direction and in the y-direction, and inFIGS. 9D and 9E, the lattice shape changes from the tetragonal latticeto the orthorhombic lattice. At the same time, the cross section of thecylinders changes to an oval shape, whereby change occurs not only inthe lattice pitch and the lattice shape but also in the refractive indexdistribution within a period (namely duty). Such changes in theperiodicity changes the photonic band structure, thereby causingvariations in the optical characteristics such as reflection andrefraction for the incident light and in the frequency characteristicsthereof. In the photonic crystal of the present invention, the change inthe periodicity may appear singly in each of the lattice pitch, thelattice shape and the refractive index distribution or in combination.

Thus, the optical element of the present embodiment can control thelattice shape or the lattice pitch of the crystal by action from theexterior, more specifically energy such as a mechanical force or anelectric field applied from the exterior, whereby an element having adesired photonic band structure can be provided with a simpleconfiguration. Since the present embodiment can be realized with asimple element configuration in which an existing photonic crystal isfixed on a substrate deformable by an externally applied energy, therecan be obtained a compact apparatus configuration. Also there is a largefreedom in selection of the shape and the material of the photoniccrystal, the photonic band structure can be regulated with a compactconfiguration. Further, since in such regulating operation the substrateitself is moved by the externally applied energy, it is possible toincrease the response speed of the element.

In the following there will be explained a second embodiment of thepresent invention.

FIG. 2 is a view showing an element configuration of an optical elementfor explaining a second embodiment of the present invention.

As shown in FIG. 2, the optical element of the present embodiment isconstituted of a photonic crystal portion (hereinafter represented as PCportion) 201 and a support portion 202. The PC 201 has amulti-dimensional periodic structure showing a periodic change ofrefractive index. A crystal structure having such multi-dimensionalperiodic structure is not particularly restricted as long as there isformed a photonic band structure capable of suppressing lightpropagation.

The PC portion 201 is incorporated in a material showing deformation inshape by externally applied energy. The PC portion 201 can be preparedby an already reported and known method, such as a lithographictechnology or an etching technology, and the method of preparation isnot particularly limited. Such preparation technology is utilized toprocess a part of the aforementioned material showing a deformation inshape by the externally applied energy. A non-processed portion is usedas a support portion 202, thereby obtaining an optical elementintegrated with the PC portion 201.

As explained in the foregoing, the optical element of the presentembodiment is constituted in a state where the PC portion 201 and thesupport portion 202 are integrated. Therefore, when the support portion202 causes deformation by externally applied energy, the PC portion 201correspondingly deforms in shape. In this case, as in the firstembodiment of the present inventin, the deformation takes place in thedirection parallel to the interfacial plane with the PC portion.

As explained in the first embodiment, since the frequency of the lightcorresponding to a photonic band structure can be determined from thelattice shape and the lattice pitch mentioned above, such change in theshape of the PC portion 201 allows to change the lattice shape or thelattice pitch thereby controlling the frequency characteristics.

Thus, the optical element of the present embodiment can control thelattice shape or the lattice pitch of the crystal by energy applied fromthe exterior, whereby an element having a desired photonic bandstructure can be provided with a simple configuration. In the presentembodiment, as a portion showing a periodic change of the refractiveindex is integrally prepared on a support portion for supporting theportion showing the periodic change of the refractive index, there canbe obtained a compact apparatus configuration. Also regulation of thephotonic band structure is rendered possible with a compactconfiguration. Furthermore, this optical element can be preparedinexpensively since it is prepared with the same material. Further,since in such regulating operation the optical element itself is movedby the externally applied energy, it is possible to increase theresponse speed of the element.

According to the present invention, there can be realized a periodicstructure controlling method allowing to control a periodic structureshowing a periodic change of the refractive index (photonic bandstructure) with a simple configuration, and an optical element havingperiodic structure control means.

In the following there will be explained examples of the presentinvention.

EXAMPLE 1

In an example 1, there will be explained an example of a configurationin which an optical element of the present example is applied to amirror. FIG. 7 is a view showing an example of a configuration of amirror in which the optical element of the present example is applied.In FIG. 7, there are shown a PC 101, a substrate 102 and a driver 501.

FIGS. 3A and 3B show a specific configuration of the PC 101 of thepresent example, employing the configuration of the first embodiment ofthe present invention.

As a constituent material, there is utilized PMMA (polymethylmethacrylate) having a refractive index of 1.49. As shown in FIG. 3A,the PC 101 is formed, by the EB lithography, in a two-dimensionalrod-shaped crystal having a honeycomb structure. However, the crystalstructure is not limited to such structure.

FIG. 3B is a cross-sectional view along the line 3B-3B in FIG. 3A. Asillustrated in these figures, the PC 101 is constituted of a rod portionshowing a periodic change in the refractive index, and a support portionfor the rods. In the present example, the support portion is madesufficiently thin, in order that the range of the deformation of the PC101 is concentrated in the support portion, thereby causing an efficientchange in the lattice pitch of the rod portion.

In the present example, a piezoelectric element is employed as thesubstrate 102. After the PC 101 is formed with the above-explainedmethod, the substrate 102 and the PC 101 are adhered to obtain anoptical element. The substrate 102, in response to a voltage signalentered from the driver 501, elongates or contracts in the direction ofthe junction plane between the PC 101 and the substrate 102. The PC 101,being closely adhered to the substrate 102, can change the shapeintegrally with the elongation or contraction of the substrate 102.

In such configuration, in a state where a light was entered from thedirection parallel to the junction plane of the PC 101 and the substrate102, the driver 501 was used to cause an elongating-contracting motionof the substrate 102 in a direction of the junction plane with the PC101 for regulating the photonic band structure so as to suppress thewavelength of the incident light, it could be confirmed that theincident light was reflected efficiently.

EXAMPLE 2

In an example 2, there will be explained an example of a configurationin which an optical element of the present example is applied to amirror. FIG. 7 is a view showing an example of a configuration of amirror in which the optical element of the present example is applied.In FIG. 7, there are shown a PC 101, a substrate 102 and a driver 501.

FIGS. 4A and 4B show a specific configuration of the PC 101 of thepresent example, employing the configuration of the first embodiment ofthe present invention.

As a constituent material, there is utilized PMMA (polymethylmethacrylate) having a refractive index of 1.49. As shown in FIG. 4A,the PC 101 is formed, by the EB lithography, in a two-dimensionalrod-shaped crystal having a honeycomb structure. However, the crystalstructure is not limited to such structure. FIG. 4B is a cross-sectionalview along the line 4B-4B in FIG. 4A. As illustrated in these figures,the PC 101 is constituted of rod portions, showing a periodic change inthe refractive index, present in an isolated manner on the substrate102.

In the present example, a piezoelectric element is employed as thesubstrate 102. In the present example, after a PMMA film coat wasapplied on the substrate 102, the above-explained method was used toobtain an optical element in which the substrate 102 and the PC 101 areadhered. The substrate 102, in response to a voltage signal entered fromthe driver 501, elongates or contracts in the direction of the junctionplane between the PC 101 and the substrate 102. The PC 101, beingclosely adhered to the substrate 102, can change the shape integrallywith the elongation or contraction of the substrate 102.

In such configuration, in a state where a light was entered from thedirection parallel to the junction plane of the PC 101 and the substrate102, the driver 501 was used to cause an elongating-contracting motionof the substrate 102 in the direction of the junction plane with the PC101 for regulating the photonic band structure so as to suppress thewavelength of the incident light, it could be confirmed that theincident light was reflected efficiently.

EXAMPLE 3

In an example 3, there will be explained an example of a configurationin which an optical element of the present example is applied to amirror. FIG. 7 is a view showing an example of a configuration of amirror in which the optical element of the present example is applied.In FIG. 7, there are shown a PC 101, a substrate 102 and a driver 501.

FIGS. 5A and 5B show a specific configuration of the PC 201 of thepresent example, employing the configuration of the second embodiment ofthe present invention. The present example is so constructed as toprovide the deforming portion, namely the support portion 202, with anelectric field substantially parallel to the periodic direction of theperiodic structure of the PC 201.

The present example employs, as the PC portion 201 and the supportportion 202, a piezoelectric element (PLZT, refractive index 2.5). Asshown in FIG. 5A, the PC portion 201 is formed, by the EB lithography,in a two-dimensional rod-shaped crystal having a honeycomb structure.However, the crystal structure is not limited to such structure. FIG. 5Bis a cross-sectional view along the line 5B-5B in FIG. 5A. Asillustrated in these figures, it is constituted of the PC portion 201 ofa rod shape, showing a periodic change in the refractive index, and thesupport portion 202.

In the optical element of the present example, electrodes 301, 302 arefurther prepared, as shown in FIGS. 5A and 5B, on the left and rightends of the support portion 202, with respect to the boundary plane ofthe PC portion 201 and the support portion 202. Since the supportportion 202 is formed of a piezoelectric element, an application ofvoltage to the electrodes 301 and 302 allows to cause deformation in thesupport portion 202. In the present example, in response to a voltagesignal entered from the driver 501, the support portion 202 elongatesand contracts in the direction of the boundary plane of the PC portion201 and the support portion 202. The PC portion 201, being integral withthe support portion 202, can change the shape with the elongation orcontraction of the support portion 202.

In such configuration, in a state where a light was entered from thedirection parallel to the junction plane of the PC portion 201 and thesupport portion 202, the driver 501 was used to cause anelongating-contracting motion of the support portion 202 in thedirection of the junction plane with the PC portion 201 for regulatingthe photonic band structure so as to suppress the wavelength of theincident light, it could be confirmed that the incident light wasreflected efficiently.

EXAMPLE 4

In an example 4, there will be explained an example of a configurationin which an optical element of the present example is applied to amirror. FIG. 7 is a view showing an example of a configuration of amirror in which the optical element of the present example is applied.In FIG. 7, there are shown a PC 101, a substrate 102 and a driver 501.

FIGS. 6A and 6B show a specific configuration of the PC 201 of thepresent example, employing the configuration of the second embodiment ofthe present invention. The present example is so constructed as toprovide the support portion 202 with an electric field substantiallyperpendicular to the periodic direction of the periodic structure of thePC 201.

The present example employs, as the PC portion 201 and the supportportion 202, a piezoelectric element (PLZT, refractive index 2.5). Asshown in FIG. 6A, the PC portion 201 is formed, by an EB lithography, ina two-dimensional rod-shaped crystal having a honeycomb structure.However, the crystal structure is not limited to such structure. FIG. 6Bis a cross-sectional view along the line 6B-6B in FIG. 6A. Asillustrated in these figures, it is constituted of the PC portion 201 ofa rod shape, showing a periodic change in the refractive index, and thesupport portion 202.

In the optical element of the present example, electrodes 401, 402 arefurther prepared, as shown in FIGS. 6A and 6B, on the upper and lowerends of the support portion 202, with respect to the boundary plane ofthe PC portion 201 and the support portion 202. The electrodes areprepared by a sol-gel method. Since the support portion 202 is formed ofa piezoelectric element, an application of voltage to the electrodes401, 402 allows to cause deformation in the support portion 202. In thepresent example, in response to a voltage signal entered from the driver501, the support portion 202 elongates and contracts in the direction ofthe boundary plane of the PC portion 201 and the support portion 202.The PC portion 201, being integral with the support portion 202, canchange the shape with the elongation or contraction of the supportportion 202.

In such configuration, in a state where a light was entered from thedirection parallel to the junction plane of the PC portion 201 and thesupport portion 202, the driver 501 was used to cause anelongating-contracting motion of the support portion 202 in thedirection of the junction plane with the PC portion 201 for regulatingthe photonic band structure so as to suppress the wavelength of theincident light, it could be confirmed that the incident light wasreflected efficiently.

1. An optical element for reflecting or transmitting an incident light,said optical element comprising a periodic structure in which refractiveindex is distributed periodicly and a deforming portion which deforms byexternal action, wherein said deforming portion is integrally arrangedwith said periodic structure along the periodic direction of saidperiodic structure, and is so constructed as to change the periodicityof said periodic structure by deforming in the periodic direction ofsaid periodic structure.
 2. The optical element according to claim 1,wherein said change in the periodicity is that in any one of the period,phase, duty and orientation of said periodic structure or in thecombination thereof.
 3. The optical element according to claim 1,wherein said deforming portion is positioned outside a path ofreflecting or transmitting light of said optical element.
 4. The opticalelement according to claim 1, wherein said deforming portion includes amember integrally joined to said periodic structure, and said memberdeforms in the direction parallel to the joining plane of said memberwith said periodic structure.
 5. The optical element according to claim1, wherein said deforming portion includes a member for supporting saidperiodic structure, and said member deforms in the direction parallel tothe plane of said member supporting said periodic structure.
 6. Theoptical element according to claim 5, wherein said member supporting theperiodic structure is the same as a member constituting said periodicstructure.
 7. The optical element according to claim 1, wherein saiddeforming portion elongates and contracts in at least one direction. 8.The optical element according to claim 1, wherein said deforming portioncauses shear deformation in at least one direction.
 9. The opticalelement according to claim 1, wherein said deforming portion isconstituted of a piezoelectric element.
 10. The optical elementaccording to claim 9, wherein said deforming portion includes a pair ofelectrodes, and said pair of electrodes are so arranged as to providesaid deforming portion with an electric field substantially parallel tothe periodic direction of said periodic structure.
 11. The opticalelement according to claim 9, wherein said deforming portion includes apair of electrodes, and said pair of electrodes are so arranged as toprovide said deforming portion with an electric field substantiallyperpendicular to the periodic direction of said periodic structure. 12.The optical element according to claim 1, wherein said periodicstructure is of a multi-dimensional photonic crystal.
 13. The opticalelement according to claim 12, wherein said periodic structure is of atwo-dimensional photonic crystal, and is composed of a portion having atwo-dimensional periodicity and a support portion for supporting theportion having the two-dimensional periodicity.
 14. The optical elementaccording to claim 12, wherein said periodic structure is of atwo-dimensional photonic crystal, and is composed solely of a portionhaving a two-dimensional periodicity.
 15. A mirror comprising theoptical element according to claim 1, and means for switching reflectiveand transmissive properties of said periodic structure alternatively byproviding said deforming portion of said optical element with externalforce.
 16. The optical deflector comprising the optical elementaccording to claim 1, and means for changing a light-propagatingdirection of said periodic structure by providing said deforming portionof said optical element with periodic external force.
 17. A controlmethod for an optical element having a periodic structure in whichrefractive index is distributed periodicly, comprising the steps ofarranging a deforming portion which deforms by external actionintegrally with said periodic structure along the periodic direction ofsaid periodic structure, and changing the periodicity of said periodicstructure by causing deformation in the periodic direction of saidperiodic structure.